Tubular heater with partial flue gas recirculation and heating method



Oct. 25, 1955 K. PERMANN 2,721,735

TUBULAR HEATER WITH PARTIAL FLUE GAS RECIRCULATION AND HEATING METHODFiled OOC. 23, 1951 2 Sheets-Sheet l His ATfornzg Oct. 25, 1955 K.PERMANN 2,721,735

TUBULAR HEATER WITH PARTIAL F LUE GAS RECIRCULATION AND HEATING METHODFiled Oct. 23, 1951 2 Sheets-Sheet 2 Fig. 2.

\veni'or= Karl Permann United States Patent TUBULAR HEATER WITH PARTIALFLUE GAS RECIRCULATION AND HEATING METHOD Karl Permann, Oakland, Calif.,assignor to Shell Development Company, Emeryville, Califi, a corporationof Delaware Application October 23, 1951, Serial No. 252,746

12 Claims. (Cl. 263-41) This invention relates to improvements incombustion furnaces for heating fluids flowed through heating tubes andto an improved method of heating such fluids, wherein the heating tubesare arranged within a heating zone or chamber so as to cause combustiongases to flow substantially longitudinally with respect to the heatingtubes, and wherein a part of the combustion gases are recirculated tothe burners. The heater and method according to the invention are, forexample, particularly useful in the carrying out of endothermic chemicalreactions, such as the dehydrogenation of alcohols or other gaseoushydrocarbons which takes place at about 1000 F., in the conversion ofethylene dichloride into vinyl chloride and hydrochloric acid at about960 F., the cracking of hydrocarbon oil, and catalytic dehydrogenationprocesses.

It is an object of the invention to provide an improved method andheater of the character described that are capable of effecting uniformand controlled heating of fluids within the tubes, wherein differentsections of the tubes are heated by separate currents of combustiongases flowing in opposite directions. In such a heater and method onesection of the tubes may be used to bring the fluids up to the desiredreaction temperature while the other section may be used to supply heatto the reaction stream while it is undergoing an endothermic reaction.

It is a further object to provide a method and heater of the characterdescribed wherein uniform and controlled heating of heat sensitivefluids is effected and the tubes containing the fluids are effectivelyshielded from flame impingements and excessive wall temperatures, whileproviding adequate heat absorption rates.

Further objects are to improve the heat distribution to the tubes bysymmetry in design and forced high rate of gas movement; to provide forflexibility in operation by using divided gas flow and partialrecirculation of combustion gas; and to effect high thermal efficiencyby providing two heating zones or chambers and discharging to the stackthe flue gas from only one of these chambers after adequate utilizationof the heat capacity of such gas.

Further objects will become apparent from the following description.

In summary, according to this invention, elongated heating tubes aremounted longitudinally within two elongated terminally adjacent heatingzones enclosed by walls and the fluid to be heated is passed through thetubes from their inlet ends to the discharge ends to traverse theheating zones in succession. Hot combustion gas is generated by one ormore burners in a separate combustion chamber, which preferablysurrounds the heating zones at the juxtaposed ends thereof andcomunicates therewith by passages permitting influx of combustion gaswith a circumferential velocity component. Combustion gas that hasalready passed through one of the heating zones is returned to thecombustion chamber, preferably tangentially so as to set up a rotatingannulus, to dilute and cool the hot combustion gas. The dilutedcombustion gas enters the heating zones at the juxtaposed ends thereofand passes as two currents in opposite directions in contact with theheating tubes towards the remote ends where the gas is withdrawn. Thepart of the divided gas stream which is withdrawn at one end is returnedto the combustion zone to dilute the hot combustion gases as describedabove, while the other part of the gas stream is discharged as flue gas.

The method and apparatus will be described in greater detail withreference to the accompanying drawing showing one illustrativeembodiment thereof, wherein:

Figure 1 is a plan view, partly in section, of a heater according to theinvention suitable for practicing the method; and

Figures 2 and 3 are transverse sections taken on correspondinglynumbered lines on Figure 1.

Referring to the drawings in detail, 10 represents a wall structure inthe form of a cylindrical shell having a horizontal axis and definingwithin itself two elongated, terminally juxtaposed heating zones orchambers A and B that are fully open to each other. The shell may belined with insulating and refractory material, as shown. The ends of theshell are fitted to end ring sections 11 and 12 which form end closuresfor the heating chambers at the remote ends thereof. The ring 11 has atangential outlet opening 13 communicating with a stack 14 for the discharge of flue gas from the chamber A. A damper 15 in the stackbreaching regulates the discharge of flue gas. The ring 12 has atangential outlet opening 16 for the discharge of flue gas from thechamber B communicating with a fan 17 for recirculating the gas, the fanbeing driven by an electric motor 18. The heating tubes 19, having fins19a on the parts thereof situated in the chamber A, extendlongitudinally through the heating chambers and axially through the endwalls of the ring sections 11 and 12; they are connected at their inletand outlet ends to headers 20 and 21, respectively. The heating tubesare preferably arranged in a circle concentric with the axis of theshell and in close proximity from the inner surface thereof but spacedtherefrom to provide room for header fittings and to permit combustiongases to circulate on all sides thereof. The heating chamber A at whichthe fluid is introduced into the tubes may be called the inlet zone andthe other chamber B the discharge zone. A gas disperser 22 or 23 ismounted coaxially within each chamber and inside of the circle ofheating tubes, supported from the shell at intervals by supports 24. Thedispersers are circular in cross section and increase progressively incross-sectional area toward the remote ends of the heating chambers, asshown, being spaced apart axially in the vicinity of the combustionchamber, which is described below. It is advantageous to provide ahelical vane 25 on the disperser 23 in order to secure a higher heattransfer effect. These dispersers may, if desired, be protected by heatresistant material, such as castable high temperature ceramic, notshown.

The shell has a peripheral opening 26 located between the juxtaposedends of the heating chambers. A pair of annular end walls 27, 28 and aperipheral wall 29 define an annular combustion chamber that is incommunication with the heating chambers through the opening 26. The wall29 has a tangential inlet 30 and a plurality of circumferentially spacedburners 31 that are preferably inclined with respect to the radii to theburners to emit hot combustion gases with a circumferential velocitycomponent in the same circumferential direction as that of the gasadmitted through the inlet 30. The helical vane 25 is also formed topermit gas rotating about the axis of the heating chamber to continuerotation in flowing toward the discharge end. The burners may be oil orgas burners, as desired; in the illustrated embodiment they are gasburners, provided with auxiliary equipment for mixing gas and air, fromsupply lines 32 and 33, respectively. Air under suitable pressure issupplied from a fan 34 driven by an electric motor 35 and fuel gas underpressure is supplied from a source, not shown, through a pipe 36 and aratio valve 37 that is connected also to the blower output to regulatethe rate of gas flow in accordance with the rate of air flow. Theburners are preferably of the premixing type emitting short flames andmay have cylindrical muffles 33. A gas return duct 39 connects thedischarge side of the fan 17 to the tangential inlet 36. Header boxes itand 1 enclose the headers 20 and 21 to retain heat.

In operation, the fluid to heated, for example, re actants capable ofundergoing an endothermic reaction, is supplied to the header 29, passedthrough the tubes 19, and discharged through the header 21. T he burners31 being in operation, the hot combustion gas generated thereby ispromptly diluted with colder t bustion gas admitted through the inlet 39to form a rotating annulus of diluted combustion gas which enters theheating zones with a circumferential velocity component through theopening 26. No flame enters the heating zones due to the ring shape ofthe combustion chamber, wherein complete mixing of hot combustion gasesand diluent gas takes place, and the tubes 19 are heated solely byconvection. One portion of the diluted combustion gas flows from theplace of introduction through the inlet zone A to the remote end thereofand is discharged through the stack 14; this portion is equal in amountto the quantity of hot combustion gas generated by the burners butconsists, of course, in part of recycled flue gas. The remaining portionof the introduced rotating diluted combustion gas flows through thedischarge zone B to the remote end thereof, whereat it is withdrawnthrough the outlet 16 and is returned to the combustion chamber by meansof the fan 17 and return duct 35; having been cooled by contact with theheating tubes l9 this recycled combustion gas is capable of reducingmaterially the temperature of the hot combustion gas upon being mixedtherewith in the combustion chamber. In flowing through the heatingchambers the rotating gases follow a generally helical path about thegas dispersers, thereby insuring rapid flow of the gases relatively tothe heating tubes, insuring higher rate of heat transfer and uniformheating of the several tubes. This helical movement is insured by thevanes which are of particular value on the disperser 23 for two reasons:Since the fan 37 induces positive circulation of the gas traversing thezone B, the vanes promote rotation of the gas about the disperser andthe pressure head of the fan is sufficient to overcome drag due to thismovement-a condition not encountered in the zone A where the disperser22 is located, unless an induced draft fan is provided in the stack, theprovision of such a fan being, of course, not excluded from the scope ofthe invention. The second reason is that, as shown in the drawing, thezone B is usually made longer than the zone A although this relation oflengths, too, is subject to variations in design as will be explainedbelow; hence, there is usually a greater tendency for the gases in thezone B to lose their rotational velocity due to drag against the heatingtubes, and this tendency is overcome by providing the vanes.

Each of the two portions of the introduced diluted combustion gasundergoes a reduction in temperature in approaching the remote end ofits respective heating zone; however, the gradients of these twocurrents are not necessarily the same and they can be controlled for anygiven rate of flow of fluid through the heating tubes at a given inlettemperature by changing the rate of heat input to the burners and therate of recirculation through the fan 17. By operating the fan 17 at ahigher rate the total quantity of gases flowing through the zone B maybe increased, resulting in a smaller temperature gradient therein;operating the burners at a hi her rate similarly reduces the gradient inthe zone A. in general, it is preferred to operate the zone A with atemperature gradient that is considQably greater than the gradient inthe other zone,

whereby the exhaust gases vented to the stack 14 are considerably coolerthan the recycled gases; this brings about improved emciency withoutrequiring that gases in contact with the heating tubes 19 in the zone Bbe at a reduced temperature.

When the heater is used for carrying out chemical reactions, the heatingtubes 19 may be regarded as comprising two sections, a preheatingsection near the inlet header 2% and a reaction section near thedischarge heador 21. The boundary between these sections is not sharplydefined but is usually in the vicinity of the combustion chamber. Hence,the relative lengths of the heating zones are selected having regard toheat transfer rates and the reaction time required.

It follows from the foregoing that control of the gas movement as toquantity and temperature is easily effected during operation byregulating the amount of heat release from the burners, the speed of therecycle fan 17 and the setting of the damper 15 in the stack-breaching.The heating gas inside the heater is held at or slightly aboveatmospheric pressure, e. g., at a gauge pressure of onehalf inch ofwater, and is advantageously neutral in composition, i. e., there iscomplete combustion of the fuel. The operation of the heater is,therefore, highly flexible.

While a specific, preferred embodiment has been shown, it is to beunderstood that changes may be made without departing from the inventiveconcept; thus, it is not in every case necessary to arrange the heatingtubes in parallel for single pass, and other known flow arrangements maybe used, and temperature control instruments can be applied to make thecontrols fully automatic.

1 claim as my invention:

1. A process for heating a fluid stream while controlling the intensityof heating independently at successive stages of the heating comprisingthe steps of flowing said stream through elongated tubes havingdifferent sections thereof contained in two separate, confined,elongated, terminally juxtaposed convection heating zones; burning fuelin a combustion zone communicating with both said heating zones at thejuxtaposed ends thereof; diluting the resulting hot combustion gas withcolder combustion gas; introducing different portions of the resultingdiluted gas into the heating zones at the juxtaposed ends thereof;flowing said portions of the diluted gas through the heating zones indirections away from said juxtaposed ends toward the remote ends thereofin contact with the sections of said elongated tubes therein to impartheat thereto; withdrawing the gas from the heating zones at said remoteends thereof; and recycling gas withdrawn at the remote end of one ofsaid convection heating zones to the combustion zone, as the said coldercombustion gas in regulated amount independently of gas withdrawn fromthe other convection heating Zone, thereby to control the intensity ofheating in the said one convection heating Zone independently of theintensity of heating in the other convection heating zone.

2. A process for heating a fluid stream while controlling the intensityof heating independently at successive stages of the heating comprisingthe steps of flowing said stream through elongated tubes havingdifferent sections thereof contained in two separate, confined,elongated, terminally juxtaposed convection heating zones; burning fuelsubstantially completely in an annular combustion zone communicatingwith both said heating zones at the juxtaposed ends thereof and exteriorthereto and thereby generating hot combustion gas; introducing coldercombustion gas tangentially into said annular combustion zone andthereby diluting the hot combustion gas and forming a rotating annulusof diluted combustion gas; introducing different portions of saidrotating annulus of diluted gas into said heating zones at thejuxtaposed ends thereof without impingement of flame on the said tubes;flowing said portions of the introduced gas through the heating zone indirections away from said juxtaposed ends toward the remote ends thereofin generally helical paths and in contact with the sections of saidelongated tubes therein to impart heat thereto; withdrawing the gasesfrom the heating zones at said remote ends thereof; and recycling gaswithdrawn at the remote end of one of said convection heating zones asthe said colder combustion gas to the annular combustion zone inregulated amount independently of gas withdrawn from the otherconvection heating zone, thereby to control the intensity of heating inthe said one convection zone independently of the intensity of heatingin the other convection heating zone.

3. A process for heating a fluid stream of reactants to reactiontemperature and thereafter supplying endothermic reaction heat theretoat a substantially uniform temperature'at an independently controlledheating intensity comprising the steps of flowing said stream ofreactants through elongated tubes having inlet and discharge sectionsthereof contained in two separate, confined, elongated, terminallyjuxtaposed convection heating zones, said tubes having inlet ends anddischarge ends at the ends of the heating zones that are remote from thejuxtaposed ends, said fluid stream being introduced into the inlet endsof the tubes; burning fuel substantially completely in a combustionchamber that is external to and communicates with said heating zones atthe juxtaposed ends thereof to generate hot combustion gas; dilutingsaid hot combustion gas with colder combustion gas; introducing aportion of the resulting diluted gas in amount approximately equal tothe amount of said hot combustion gas generated into the heating zonethat contains the inlet sections of the tubes at the juxtaposed endsthereof; flowing said portion through said heating zone in contact withthe inlet sections of the elongated tubes and thereby heating saidreactants to reaction temperature; withdrawing said portion of gas fromthe said heating zone at the remote end thereof; introducing theremaining portion of the diluted gas into the other heating zone at thejuxtaposed end thereof; flowing said remaining portion of the gasthrough the latter heating zone in contact with the discharge sectionsof the elongated tubes and thereby supplying endothermic reaction heatto said reactants; withdrawing the said remaining portion of the gasfrom the latter heating zone at the remote end thereof; returning thelatter withdrawn portion of gas to the combustion zone as the saidcolder combustion gas; and regulating the amount of said remainingportion of the diluted gas to control the intensity of heating of thesaid other convection heating zone independently of the intensity ofheating of the inlet ends of the tubes.

4. The process according to claim 3 wherein the said colder'combustiongas is introduced into the combustion gas in a tangential direction toset up a rotating annulus of diluted combustion gas; both of saidportions of diluted gas are introduced into the respective heating zoneswith a rotary motion directly from the inside of said rotating annulus;and both of said portions of diluted gas are flowed through therespective heating zones with substantially helical motions.

5. A heater for fluid material comprising an enclosing wall defining apair of terminally juxtaposed, elongated convection heating chambers; aplurality of elongated heating tubes situated within and extendinglongitudinally through both said chambers for the passage of said fluidmaterial; a combustion chamber situated at the terminally juxtaposedends of said chambers and communicating with both said chambers at saidends thereof; one or more burners in said combustion chamber forgenerating hot combustion gas; outlets situated at the ends of theheating chambers that are remote from said juxtaposed ends forwithdrawing combustion gases at said remote ends; a gas return ductinterconnecting the outlet of one of said heating chambers to thecombustion chamber for returning gas withdrawn at the remote end of saidheating chamber to dilute and cool the hot combustion gases; and meansfor regulating the flow of said withdrawn gas through the gas returnduct independently of gas withdrawn from the other of said heatingchamber, for controlling the intensity of heating in said one heatingchamber independently of the intensity of heating in the said otherheating chamber.

6. A heater according to claim 5 wherein the heating chambers are ofunequal lengths, the heating chamber having the outlet thereof connectedto said return duct being longer than the other heating chamber.

7. A heater for fluid material comprising a generally cylindricalenclosing wall defining a pair of terminally juxtaposed, elongatedheating chambers; a plurality of elongated heating tubes situated withinand extending longitudinally through both said chambers for the passageof said fluid material; an annular combustion chamber situatedperipherally about said wall at the juxtaposed ends of the heatingchambers and communicating with both heating chambers at said endsthereof through one or more passageways permitting the entry of gas intothe heating chambers with a circumferential velocity component; one ormore burners in said combustion chamber for generating hot combustiongas; a tangential inlet for colder combustion gas for said combustionchamber whereby colder combustion gas introduced therethrough willpromptly dilute and cool said hot combustion gas and form a rotatingannulus of diluted combustion gas in the combustion chamber; outletssituated at the ends of the heating chambers that are remote from saidjuxtaposed ends for withdrawing combustion gases at said remote ends;and means for returning gas withdrawn at the remote end of one of saidheating chambers to said tangential inlet.

8. A heater according to claim 7 wherein said combustion chamber has aplurality of burners, each burner being inclined with respect to theradius to the respective burner so as to discharge hot combustion gaswith a circumferential velocity component in the circumferentialdirection in which said colder combustion gas is admitted into thecombustion chamber through the tangential inlet.

9. A heater according to claim 7 wherein the combustion chamber has aperipheral wall and two annular ends walls, each end wall extendingradially outwardly from one juxtaposed end of a heating chamber, saidheating chambers being fully open to each other at said ends thereof,the combustion chamber open to the heating zones between the innerportions of said annular end walls about the entire periphery and saidburners are of the type producing short flames, whereby the diluted gasadmitted into the heating zones is substantially free from flame.

10. A heater for fluid material comprising a generally cylindricalenclosing wall defining a pair of terminally juxtaposed elongatedconvection heating chambers that are fully open to each other at thejuxtaposed ends thereof; a plurality of elongated heating tubes situatedadjacent said wall Within and extending longitudinally through both saidchambers for the passage of said fluid material; a peripheral opening insaid enclosing wall between the juxtaposed ends of the heating chambers;a pair of annular walls extending outwardly from said opening andconnected by a peripheral wall spaced outwardly from the enclosing walland defining an annular combustion chamber; a tangential inlet forcolder combustion gas in said peripheral wall; a plurality ofcircumferentially spaced burners in said combustion chamber forgenerating hot combustion gas; outlets situated at the ends of theheating chambers that are remote from said juxtaposed ends forwithdrawing combustion gases at said remote ends; means including areturn duct and a blower for returning gas withdrawn at the remote endof one of said heating chambers in regulated amount independently of gaswithdrawn from the other chamber to said tangential inlet for dilutingand cooling the hot combustion gases and forming a rotating annulus ofdiluted combustion gases within the combustion chamber, whereby theintensity of heating in said one heating chamber can be controlledindependently of the intensity of heating in the said other heatingchamber; and a gas disperser within at least one heating chambersituated substantially at the axis thereof radially inwardly from theheating tubes.

11. A heater according to claim 10 wherein the gas disperser is circularin cross section and is situated in the heating chamber, the remote endof which is connected to said return duct, said disperser having ahelical vane at the periphery thereof.

12. A heater according to claim 10 wherein the crosssectional area ofthe gas disperser increases progressively from the juxtaposed end of theheating chamber to the 15 remote end thereof.

References Cited in the file of this patent UNITED STATES PATENTSChoinski Sept, 13, 1927 Newhouse Mar. 19, 1929 McCann June 11, 1929Besta July 2, 1929 Leamon Oct. 31, 1933 Ramseyer May 18, 1948 Huber Apr.17, 1951 Lorenzo June 5, 1951 Lacerenza Mar. 25, 1952 Carson July 20,1954 FOREIGN PATENTS Great Britain 1904

